The present invention relates generally to molding of glass optics and, more particularly, to the fabrication of molding tools by coining methods at elevated temperatures, and the use of such tools in the molding of glass lenses and lens arrays.
Coining methods have long been used to reproduce features onto surfaces for a variety of applications. For example, U.S. Pat. No. 4,243,618 to Van Arnam describes a method for forming retroreflective sheeting having a plurality of retroreflective cube-corner prisms distributed over one of its surfaces such that the prisms are disposed in a planar array. The method comprises clamping a plurality of pins together such that the ends of the pins form a substantially planar surface, in scribing the planar surface for forming thereon a continuous pattern of solid trigonal pyramids with internal dihedral angles of ninety degrees, releasing the bundle of pins and rotating the individual pins for changing the angular orientation of the formed trigonal pyramids on adjacent pins re-clamping the pins together and using the inscribed surface of the bundle of pins for forming a mold, containing cube-corner prism cavities, and producing prismatic retroreflective sheeting by embossing, molding or casting in such mold. Van Arnam focuses on eliminating problems associated with orientation and the creation of a planar surface from the pre-assembled array. The materials being molded here are either monomer or polymer in nature, and the embossing is done at or near room temperature (20xc2x0 C.). Suitable mold tool materials listed are xe2x80x9ccopper, brass, aluminum, hard plastic, hard rubber, and the like.xe2x80x9d The coining tool is an array of individual tools, each one having an optical surface machined onto its surface. The act of holding a large group of pins together to maintain a planar surface of any accuracy that is determined by aligning the vertex points from a number of individual spheres presents a considerable problem. Also, the need to reliably machine identical precision features into a number of tools adds considerable cost and effort to the process.
U.S. Pat. No. 5,623,368 to Calderini describes a method for the manufacture of a micolens sheet in which a plate of deformable optical material is pressed against an undeformable furrowed surface in such a manner that neither the convex surfaces of the microlenses nor the surface of the plate of optical material that is opposite the one that bears the microlenses enter into contact with surfaces able to alter them. In essence, the lens surfaces are free formed in this stamping operation which controls only the overall size and shape of the lens. This manner of forming the lens surface precludes the formation of complex shapes such as aspheres, torics, or other desired geometries. The mold tool itself is manufactured by conventional engraving and masking techniques.
U.S. Pat. No. 5,298,366 to Iwasaki et al. describes a method of producing a microlens array and the necessary tooling, and in particular, the inverted master tool used in the process. The mold tool is made of a resist material and the optical surfaces are formed by heating an intermediate material and thereby smoothing the surfaces of the projections. The mold tool is then used to stamp out the finished lens arrays. Again, the surfaces are formed by inexact methods and rely on surface tension between the material and its surroundings to generate the optical form. This optic can only be spherical at best, and the materials used would not endure very high temperatures.
The above patents state either directly or implicitly that their intended use is with plastic materials or a suitable low melting point glass. This is also evidenced by their choice of tooling materials, most of which could not withstand the high temperatures encountered in molding high temperature glasses without experiencing some sort of degradation. Also, in each of these methods where coining or stamping is used to produce the mold tool, a problem arises that is symptomatic to every stamping operation. A basic physical law is that of conservation of mass, which here suggests that when a volume of material is displaced from one region of an object, a comparable volume of material must appear in another region. For homogeneous materials of constant stiffness, this naturally occurs at a point near the displacement, which in the case of forming small microlenses, is evidenced by a ridge or mushroom effect around the circumference of the impression. This may be overcome by pressing down to a flat portion on the coining mandrel and exerting a high level of force to displace the material away from the feature. The problems here are twofold. First, the force needed to planarize the piece may be excessive and cause other problems such as high internal stresses to develop in the tool. Also, the set up of such a tool is very costly if the depth of the feature is to be held with any precision, as is generally required in optical applications. One way to overcome this mushrooming problem is to planarize the mold tool through a secondary operation after the coining is done, but this is expensive as well and may cause material to flow back into the feature upon machining. Therefore, an effective and economical method for manufacturing mold tools with complex optical features is needed that will withstand the harsh environmental conditions associated with high temperature glass molding and will replicate without flaw the precise geometries required.
It is therefore an object of the present invention to provide a method for the fabrication of a molding tool that contains high precision optical features for molding arrays of optical elements.
It is a further object of the present invention to provide a method for the fabrication of a molding tool that can be used for molding high temperature glass optics.
Yet another object of the present invention is to provide a method for the fabrication of a mold tool by coining which obviates mushrooming of the surface of the mold tool.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon the reading of the description, claims and drawings set forth herein. These features, objects and advantages are accomplished by forming a molding tool blank out of a suitable glass, providing an optical quality polished surface on the face of the molding tool blank whereon the optical features are to be impressed, forming an indenter tool or punch with a predetermined optical surface geometry which is the negative of the optical features/elements to be formed with mold tool, coating the optical surface of the indenter tool or punch with a release coating, creating an axial viscosity gradient in the tool blank by heating the mold tool blank in order to generate an axial thermal gradient therein, pressing the indenter tool or punch into the heated mold tool blank to thereby form a desired optical feature in the surface of the mold tool blank, cooling the tool blank and removing the indenter tool or punch from the mold blank. The process steps of creating an axial viscosity gradient in the tool blank by heating the mold tool blank, pressing the indenter tool or punch into the heated mold tool blank to thereby form a desired optical feature in the surface of the mold tool blank, cooling the tool blank, and removing the indenter tool or punch from the mold blank are practiced in a non-oxidizing environment. As mentioned above, each indenter tool is fabricated to have the negative of a predetermined optical surface geometry. That geometry may be spherical, aspherical, or an otherwise complex geometry. If it is desired to produce a mold tool for molding an array of integrally formed optical elements, the indenter tool or punch or the tool blank is repositioned and the axial viscosity gradient in the tool blank is re-established. The indenter can then be pressed again into the surface of the tool blank. In such manner, a plurality of identical optical features can be produced in the surface of the tool blank. Or, alternatively, different indenter tools can be used to form an array of individual and/or different optical features in a single mold tool surface. In either case, a mold tool formed with the method of the present invention can be used to mold integral arrays of optical elements which can then be cut into individual optical elements as desired.
The method of the present invention uses a vitreous material for the mold tool. The viscosity of the vitreous material is dependent on some other parameter that can be regulated. By controlling the viscosity gradients in the vitreous material, the shape of the feature being generated can be closely controlled without adding secondary operations such as planarizing after the forming process in order to provide a planar surface where the forming process has caused deformation to occur. This is usually done by grinding and polishing of the glass, which can add considerable cost to the tool. The present invention uses differential heating of the mold tool blank to thereby control its viscosity along an axial direction and consequently eliminate some of the problems noted above. With the present invention, the displaced material is caused to flow away from the impression point and to a region where the viscosity is low enough to permit fluid movement. This is achieved by causing the temperature at the base of the tool to be higher than the temperature at the insertion point. The viscosity must be low enough at the point of insertion so as to allow replication of the glass without chipping, yet high enough to cause the material to flow away from the lens-forming region. However, if the viscosity is too low at the insertion point, the material will sag or be pulled in by adhesive forces as the glass encounters the coining tool, which also results in unacceptable deformation in the region immediately surrounding the desired features. The present invention again differs from the aforementioned works since it can be performed at an elevated temperature (xe2x89xa11000xc2x0 C.) depending on the glass used. Proper material choice is essential to success, such that the transformation temperature of the mold tool is sufficiently greater than the molding temperature of the finished product. Also of great importance is the application of appropriate coatings to the mold tool that will adhere to the tool at high temperatures while also acting as a release coating to the glass being molded into lenses.
The method of the present invention is intended for fabricating mold tools for molding micro-optical elements. The term xe2x80x9cmicro-optical elementsxe2x80x9d as used herein is intended to mean optical elements such as lenses having a maximum diameter of not more than about one millimeter. Thus, the indenter tool is used to form depressions in the mold tool preform where the individual depressions have a maximum diameter of not more than about one millimeter. In addition, the depressions formed should have a depth/diameter ratio of not more than about 0.2. For example, if a depression is formed having a diameter of about 350 microns, then the depth of the depression should be no more than about 70 microns.